Peter Watson’s The Great Divide: History and Human Nature in the Old World and the New begins on a particularly inflammatory note. Taking account of the critical reaction that followed a 2009 exhibit at the British Museum on Montezuma and Aztec culture, Watson notes how some recoiled at the ferocity of a civilisation where human sacrifice was a central practice. Philip Hensher, the novelist and critic, was outraged: “If there is a more revoltingly inhumane and despicable society known to history than the Aztecs, I really don’t care to know about it,” Hensher sniffed. “It is difficult to imagine a museum display that gives off such an overwhelming sense of human evil as this one.” Watson himself, whose previous works include The German Genius and Ideas: A History, does not frame his argument in such melodramatically moral terms, but all civilisations have deployed the same stark discourse. When the Spanish conquistadors encountered the Aztecs in the early 16th century they were revolted at what they saw in Tenochtitlan, the Aztec capital, calling it “a city of pyramids and sacred temples that reeked with the blood of human sacrifice”. The conquering Spaniards treated the Aztecs with few tender mercies: they may have recoiled at what they saw, but they did not stint on the use of violence in their imperial conquest. Watson is less interested in the morality of human sacrifice than why the Aztecs used human victims in the first place. Looking at a vast swath of history – circa 15,000BC to 1500AD – he outlines a set of distinctions and contrasts that separate Eurasia, what Watson dubs the Old World, from the cultures of Mesoamerica and North America, the New World. Ransacking the specialist literature from a collection of disparate fields – cosmology, climatology, geology, palaeontology, mythology, botany, archaeology and volcanology – Watson considers how ecology, broadly construed, shaped the evolution of human civilisation. He owes a considerable debt here to Jared Diamond, whose book Guns, Germs and Steel: A Short History of Everybody for the last 13,000 years, was an unlikely blockbuster in the late 1990s and started a trend for big picture histories that look at long-term climatic shifts as decisive factors of historical change. Watson doesn’t have Diamond’s catchy three-word formulation; but he similarly argues that geography, climate and weather are inextricably bound to destiny. “The physical world,” Watson writes, “which early people inhabited – the landscape, the vegetation, the non-human animal life, plus the dominant features of the climate, of latitude and the relation of the land to the sea, determined the ideology of humans, their beliefs, their religious practices, their social structure, their commercial and industrial activities, and that, in turn, ideology. Once it had emerged and cohered, determined the further characteristic interaction between humans and the environment.” This is a typically bold statement in a book that is not short on rhetorical bravado. Watson comes perilously close to a kind of one-size fits all explanation. He paints in very bold, broad strokes that leave the mind reeling: Eurasia is a universal term that takes in cultures as disparate as Han Chinese and Pharaonic Egypt, Roman and Assyrian. But where is Islam? Watson has precious little to say about the spread of Muslim civilisations and his explanations often devolve into a crude environmental determinism that, he suggests, explain everything from the advent of monotheism to the rise of democracy. Watson does not simply present his data, he hurls it at his readers with manic gusto. He flies over time and space, throwing out observations here and nuggets of information there. We learn about “alcohol-based hospitality”; the role of roots and tubers; the sexual mutations of ancient gods. We are treated to eccentric chapters on “The Psychoactive Rainforest and the Anomalous Distribution of Hallucinogens” and “Shaman-Kings, World Trees and Vision Serpents”. The Great Divide is, among others, a grab bag of historical trivia. There is much speculation here, and too often a lack of conclusive evidence. Watson sees broad climatic factors as shaping forces of culture in each hemisphere. The dominant feature of the Old World was the “weakening monsoon”, which brought drying trends to the Eurasian land mass. This, in turn, led to seasonal fluctuations, which provoke the rise of fertility cults. The Old World gave rise to the cultivation of cereal grasses; domesticable animals were used to plough fields and transport goods. Pastoral nomadism spread language and technology. As Watson notes, Eurasia is geographically orientated on an east-west axis. Climates are less varied there, animals and goods could move around with ease. Watson correlates religious mythology with natural events; the great Biblical flood, for example, might be traced to rising sea levels around 6000BC. In the Americas, there were few such animals; primary foodstuffs grew year round, in marked distinction to cyclical Old World cereal crops. The land mass of the Americas was orientated on a north-south axis, with its major civilisations – Chavin, Moche, Olmec, Maya, Inca, Aztec – concentrated in the tropics. Violent weather brought about by El Niño, which unleashed freakish storms and winds on Mesoamerica. Central America is also at the juncture of several tectonic plates; earthquakes and volcanoes also wrought great damage. Gods were invoked to stave off the devastation, with little success. Watson argues that the major civilisations of the New World were typified by a “more vivid religion” marked by shamanism and the use of psychoactive drugs to produce visionary hallucinations (the Aztecs used a mushroom called teonanacatl to produce temixoch, the “flowery dream”). and Watson writes “the sheer vividness, and the fearsome nature of some of the transformations experienced in trance, the overwhelming psychological intensity of altered states of consciousness induced by hallucinogens, would, among other things, have made New World religious experiences far more convincing and therefore more resistant to change than those of the Old World.” This is a peculiar line of speculation. Watson hopes to rescue the Aztecs from the contempt and condescension of posterity; but his analysis has a whiff of old-fashioned exoticism to it. His theories about human sacrifice are likely to be more controversial. Human sacrifice in the New World persisted longer after the practice waned in Old World cultures. Watson takes up much space trying to account for why human sacrifice was a central feature of New World cultures. He contends, “the most profound and revealing difference between the Old World and the New occurs in the realm of human sacrifice”. Here, he explains, the Old World differed from the new because of the prevalence of domestic animals. Human sacrifice was, over time, superseded by animal sacrifice; and, ultimately, blood sacrifice “was abolished altogether”. Not so in the New World. Watson notes that human sacrifice became more widespread by the 15th century. Aztecs were sacrificing many thousands of victims every year. What accounts for what now seems a barbaric practice? The author falls back on natural events to explain the endurance of human sacrifice. Volcanoes did not stop erupting; earthquakes shattered the villages and cities of Inca and Aztec civilisation. Appeals to gods became more and more elaborate. Death became theatre. Shamans consolidated their power by going into trance. Other worlds beckoned, and death promised a delivery.